1. Field of the Invention
This invention relates to a fluorescent lamp and, more specifically, to a fluorescent lamp in which an aperture portion is formed in a fluorescent layer along an axial direction of a tubular bulb so that light from the fluorescent lamp is radiated concentrically in one direction, for example, to be used as a reading light source for facsimiles.
2. Background Art
FIGS. 5 to 7 show the configuration of this type of aperture type fluorescent lamp of the prior art. FIG. 5 shows an aperture type fluorescent lamp 90 configured such that a thermo-electrode or cold cathode is provided at both ends of a tubular bulb 91. In this case, an aperture portion 95 is generally formed in a fluorescent layer 94 formed on an inner surface of the tubular bulb 91 by coating the entire inner surface thereof with the fluorescent layer and removing a part of the fluorescent layer 94 in an axial direction of the tubular bulb 91.
With this configuration, light directing toward the inside of the tubular bulb 91 out of light emitted from the fluorescent layer 94 is radiated outward from the aperture portion 95 which lets light passing therethrough directly. Thereby, the aperture portion 95 is brighter than other portions, and the aperture portion 95 is turned toward a direction to be illuminated.
FIG. 6 shows an example of an aperture type fluorescent lamp 80 configured such that lighting is carried out by a pair of external electrodes 82, 83 provided on the outer surface of the tubular bulb 81 and facing each other. Also in this case, an aperture portion 85 is formed in the fluorescent layer 84 formed on the inner surface of the tubular bulb 81 by the same manufacturing manner as described above, and the same function as that of the prior art described above is obtained.
In the case of this aperture type fluorescent lamp 80, the external electrodes 82, 83 are formed by aluminum vapor deposition or silver conductive coating to have a reflection function, thereby making it possible to reflect light going to the outside of the tubular bulb 81 out of light emitted from the fluorescent layer 84 toward the inside and radiate it from the aperture portion 85. Thus, efficiency can be improved.
Further, FIG. 7 shows an example of an aperture type fluorescent lamp 70 configured such that lighting is carried out by an internal electrode 72 provided at the center in an axial direction of a tubular bulb 71 and an external electrode 73 provided on the outer surface of the tubular bulb 71. Also in this case, an aperture portion 75 is formed in a fluorescent layer 74 formed on the inner surface of the tubular bulb 71 by the same manufacturing manner as described above and the same function as described above is obtained.
However, in the aperture type fluorescent lamps 70, 80, 90 of the prior art described above, the aperture portions 75, 85, 95 are simply formed in the fluorescent layers 74, 84, 94 formed on the inner surfaces of the tubular bulbs 71, 81, 91, respectively. If a reflection function is given to the outer surface of the tubular bulb, the shape of means having such a reflection function is limited to the shape of the outer surface of the tubular bulb 71. Therefore, desired distribution characteristics cannot be provided to light radiated from the aperture portions 75, 85, 95, resulting in nonuniform illuminance.
Particularly, in the aperture type fluorescent lamp 70 having the internal electrode 72 shown in FIG. 7, as the internal electrode 72 shades the emitted light, the above nonuniform illuminance becomes more significant and the internal electrode 72 can be seen from the aperture portion 75 directly, thereby giving an observer a feeling of disorder.